Innate immune responses permit strong resistance to infection during the hours and days that follow inoculation of microbes. The innate immune system is hard wired in the sense that it depends upon germline-encoded receptors to recognize microbes, and signal transduction pathways to elicit the genetic and biochemical responses that restrict infection. Within our species, inter-individual differences in susceptibility to infection likely reflect differences in innate immune performance, based on differences in genetic mal<eup. Sometimes these differences are extreme (i.e., severe immunodeficiency disease may result from some mutations); sometimes the differences are rather subtle, or manifested only in susceptibility to a small collection of infectious organisms. It is imperative to know the full complement of genes that are necessary and sufficient for host resistance, however we measure it. An excellent start has been made, in that we now know many of the receptors that sense infection, and the ligands that activate these receptors. This proposal, tightly integrated with the other components of the U19 grant, extends a classical genetic approach, already used with great success to delineate the signaling pathways that comprise our innate immune system. It is empowered by recent advances in two areas: 1) DNA sequencing, which has completely transformed the process by which mutations are identified, and 2) IPS cell technology, which permits the recapture of mutations made in primary somatic cells to the germline for genetic mapping and extended study in the context of a complete organism. Here we describe how we will use ENU mutagenesis in the mouse to screen to near-saturation in pursuit of every molecular component of the innate immune system. As these components are discovered, we will operate in close collaboration with our colleagues at SBMI, ANU, Stanford, and TSRI to understand how each element works in the context of immune signaling, and to determine the effect of each mutation on adaptive immunity, and development of the immune system. We will also study the roles played by the corresponding genes in human immune cells. Closely tied to (and dependent on) the Genetics Core Laboratory, our program will create a valuable resource for the biomedical research community, in that functionally null alleles will be created at the majority of loci in mice and archived as sperm for quick accessibility. Moreover, we will donate all mutations that yield an immunological phenotype to the MMRRC for distribution.